Defect-mediated intersystem crossing in poly(heptazine imide) for singlet oxygen-accelerated anthraquinone redox cycling

This study demonstrates that strategically leveraging singlet oxygen (¹O₂) through defect-engineered intersystem crossing (ISC) significantly enhances photocatalytic H₂O₂ production, achieving unprecedented performance with anthraquinone (AQ, 5 %)-modified poly(heptazine imide) (AQ₅PHI). Spectroscopic, electrochemical, and computational analyses reveal that the electron-donating capability of defect sites optimizes the activation of triplet oxygen (³O₂) into ¹O₂, a crucial intermediate in the AQ-mediated redox cycle. Density functional theory (DFT) simulations further support this mechanism, demonstrating that ¹O₂ significantly decreases the activation energy barrier for anthrahydroquinone (AHQ) oxidation from −0.29 to −1.30 kcal mol⁻¹ compared to ³O₂. The proposed mechanism is further validated through both theoretical calculations, such as DFT and experimental evidence. AQ₅PHI achieves a H₂O₂ generation rate of 76.8 mmol g⁻¹ h⁻¹ under O₂-saturated conditions, highlighting the strong dependence of ¹O₂ generation on O₂ availability for accelerating AQ redox cycling. Furthermore, radical scavenging experiments and electron paramagnetic resonance spectroscopy, combined with phosphorescence spectroscopy, confirm the predominance of ISC-mediated energy transfer pathways over hole-mediated oxidation.